The present invention is advantageously applied to a radio frequency communication system such as that described in co-assigned U.S. Pat. No. 5,530,702 ("the '702 Patent") to Palmer et al. which is hereby incorporated by reference. In the system described in the '702 Patent, an interrogator transceiver transmits a BEGIN signal to a group of independently operating data terminals, for example, radio frequency identification (RFID) tags, whereupon each RFID tag transmits an appropriately timed request to transmit (RTT) signal to the interrogator and, when the RTT signal is acknowledged, the RFID tag transmits a data containing signal which is received by the interrogator. As set forth in the system described in the '702 Patent, RFID tags are oriented and positioned randomly with respect to the interrogator at distances which vary by one or more orders of magnitude. As a consequence, the signal strength of transmissions received by the interrogator from more closely positioned RFID tags varies from the received signal strength of transmissions from more distantly positioned tags by several orders of magnitude.
As disclosed in the '702 Patent, each RFID tag transmits digital signals to the interrogator using two-level binary pulse width modulation (PWM), in which logical zeroes are represented by short duration pulses (nominally 33% duty cycle) timed at the beginning of each clock cycle (illustrated, by example, in FIG. 1a), and logical ones are represented by long duration pulses (nominally 67% duty cycle) also timed at the beginning of each clock cycle, as illustrated in FIG. 1b. These digital PWM signals are modulated onto a radio frequency (RF) carrier for transmission to the interrogator transceiver. In the interrogator transceiver (FIG. 2), each transmitted RF signal is demodulated to produce a signal containing pulses of long and short duration. The pulses are provided to a pulse detector which detects a logic state, i.e. "0" or "1", for each pulse, based on the duration of the pulse in relation to a "critical duty cycle" setting of the transceiver. In the exemplary embodiment disclosed in the '702 Patent, the critical duty cycle of a pulse detector of the transceiver is set to 50%, such that pulses having a duty cycle less than 50% are determined to represent the value "0" and pulses having a duty cycle which is greater than 50% are determined to represent the value "1".
In the system disclosed in the '702 Patent, each RFID tag responds at a pseudo-randomly determined time to the BEGIN signal transmitted by the interrogator; the interrogator does not specify in advance or have prior information as to which RFID tag will transmit signals in turn. Therefore, the receiver settings of the interrogator cannot be adjusted prior to receiving the RTT signal from each RFID tag in turn. In the receiver circuitry of the transceiver, illustratively shown in FIG. 2, a demodulator 22 is followed by a voltage comparator 24 for producing a sequence of pulses each having a duty cycle determined by the time in which the signal level lies above a reference voltage Vref 26. As indicated in FIGS. 3a and 3b if the receiver settings of the interrogator transceiver remain the same for receiving both relatively weak and relatively strong signals from RFID tags, the duty cycles of the PWM pulses of the relatively strong signals appear to grow longer, as indicated in FIG. 3b. If no adjustment is made for receiving relatively strong signals, the duty cycle of short duration pulses representing the value "0" could exceed the critical duty cycle (e.g. 50% duty cycle) used to distinguish short duration pulses from long duration pulses and cause the short duration pulses to be indistinguishable therefrom. Accordingly, prior to the herein described invention a need existed for a receiver of PWM signals to adjust such critical duty cycle based on determining an actual duty cycle at which either short duration or long duration pulses are received.
The prior art known to Applicants does not teach a system which fills this need. U.S. Pat. No. 4,695,840 to Darilek ("the Darilek Patent") describes a decoder which determines the duty cycle of a transmitted pulse, which duty cycle encodes a multiple symbol information signal corresponding to the settings of multiple switches in a transmitter. However, the Darilek Patent is only concerned with a single transmitted pulse and does not adjust receiver settings for detecting further transmitted pulses. Moreover, the Darilek Patent does not describe a system which uses a critical duty cycle to distinguish between pulses having different duty cycles.
U.S. Pat. No. 5,471,187 to Hansen et al. ("the Hansen Patent") describes a digital RF transceiver (See FIG. 12) which incorporates a voltage-referenced comparator 305 for determining the transitions between positive and negative pulses of a data signal which is not pulse width modulated. In that described transceiver, the reference voltage level 322 of the comparator 305 is varied in accordance with the strength of the received signal. Not being concerned with the particular problems of receiving pulse width modulated signals, the Hansen Patent does not describe measuring a duty cycle of transmitted pulses or adjusting a critical duty cycle for distinguishing between different transmitted pulses.
Accordingly, it is an object of the invention to provide a system in a receiver which determines a duty cycle at which either first or second pulses of an initially transmitted pulse sequence is received and adjusts a critical duty cycle of a pulse detector in accordance with the duty cycle which is determined.
Another object of the invention is to provide a system in a receiver which determines the duty cycle of pulses by determining correspondence between the information detected therein and reference information at the receiver.
Still another object of the invention is to provide a method for improving detection of a digital PWM signal by determining the duty cycle of either first duration pulses or second duration pulses in an initially transmitted sequence, adjusting a critical duty cycle in the receiver in accordance with the duty cycle which is determined and detecting a further transmitted sequence in accordance with adjusted critical duty cycle.